1. Field of the Invention
This invention relates to zoom lenses for photographic purposes, more particularly, but not exclusively, for video movie cameras.
2. Prior Art
U.S. Pat. Specification No. 4,749,267, discloses a zoom lens including twelve elements arranged in four optical units. There is a positive unit facing the long conjugate and movable for focussing. There is a negative unit movable for zooming. There is a second negative unit which is movable for zooming and there is a rear unit which is stationary. The positive unit facing the long conjugate consists of a cemented doublet followed by a single element. The negative unit consists of a negative element and a negative doublet concave towards each other. The second negative unit consists of a single component including one negative element. The fourth unit consists of a front sub-unit and a rear sub-unit, wherein the front sub-unit consists of two positive lens elements followed by a negative lens element and the rear sub-unit consists of either one or two positive lens elements. This zoom lens has a zoom range of only 3 X and a f-number of about f/2.0.
There have been several proposals for zoom lenses in which the first three units are similar to those of U.S. Pat. No. 4,749,267, but which use different rear units. For example, in U.S. Pat. No. 4,572,620 the rear unit includes eight elements. The zoom lens of U.S. Pat. No. 4,572,620 has a zoom range of 5.6 and a f-number of about f/1.4.
U.S. Pat. No. 4,281,906, also shows a 4-unit zoom lens with a front positive unit for focussing action followed by two negative zooming units and a positive stationary rear unit. However, this zoom lens consists of 17 to 20 lens elements, with the last 7 to 10 elements comprising the Positive rear unit. That known lens has a zoom ratio of 12.65 with an f-number of f/1.6.
It is an object of the present invention to provide a zoom lens having, in combination, a higher zoom ratio, superior f-number and fewer elements than the zoom lenses of the prior art.